1. Field of the Invention
The present invention relates to a sample evaluating method for evaluating defects or the like of a sample by irradiating a cyclically intensity-modulated excitation beam to the sample and by measuring the thermal expansion displacement resulting therefrom on the surface of the sample.
2. Description of the Prior Art
In the case that an excitation beam which has been cyclically intensity-modulated is irradiated to a sample, the sample is heated by absorption of the excitation beam and is thermally expanded thereby. As the excitation beam and is cyclically intensity-modulated, the temperature changes of the sample on account of heating are made cyclic, thereby causing the sample to be thermally expanded and displaced. Such a method for evaluating a sample by measuring the thermal response has been known as the photoacoustic method.
FIG. 8 shows a method for measuring the thermal expansion displacement of a sample by use of the Michelson laser beam interference method (Miranda, Applied OPTICS Vol. 122, No. 18, P2882 (1983)). Herein, 61 indicates a sample to be measured, 62 indicates an excitation beam source to give thermal expansion displacement to the sample, and the beam which comes from the excitation beam source 62 is intensity-modulated by a chopper 63 and is irradiated to the sample 61. The thermal expansion displacement is metered by a laser interference method. Therefore, the beam coming from a measuring laser 64 is divided into two by a translucent mirror 65, one beam of which is irradiated to the point of thermal expansion measurement of a sample and the other of which is irradiated to a fixed mirror 66. Then, the reflection beam therefrom is interfered and is received by a photoelectric converter 67. The electric output E from the photoelectric converter 67 is arithmetically processed by an expression (1). EQU E=C.sub.1 +C.sub.2 Cos (P.sub.(t) +.phi.) . . . (1)
Here, C1, C2 and .phi. are constants which depend upon the composition of a sample 61 and an interferometer and upon photoelectric conversion coefficients, and .lambda. is the wavelength of a measuring laser. P(t) is a phase change by surface displacement of a sample due to thermal expansion displacement resulting from irradiation by the excitation beam, and the thermal expansion displacement of a sample is metered by this measurement, thereby causing the thermo-elastic characteristics thereof to be evaluated. FIG. 9 shows a method based on the reflectivity measuring method (Refer to the Japanese Pat. Laid-Open Gazette No. Sho-61-2046). The beam which comes from the excitation laser 30 is cyclically intensity-modulated by a modulator 32 and is irradiated to a sample 22, thereby causing the sample to cyclically produce temperature changes. The temperature changes cause changes in the optical reflectivity of the sample. In order to detect changes in the optical reflectivity, measuring laser 50 is irradiated to the temperature change measuring point of a sample (the same position as the point of excitation laser irradiation in this Figure) through a mirror 36, and the reflection beam thereof is detected by a photodetector 56. The change of reflectivity is obtained by a signal processing circuit 58 from this output.
In the method for measuring the thermal expansion of a sample by the former Michelson laser beam interference, changes of the constants C.sub.1 and C.sub.2 in the former expression (1) result in a lowering of the measurement accuracy as disturbance.
For instance, there are cases in which the reflectivity of a sample changes due to temperature changes of the sample caused by irradiation of an excitation beam and due to changes of the plasma (electrons, holes) density (in case of semiconductor sample). In this case, as signals of an interference beam include disturbance signals accompanying the changes of reflectivity, it is impossible to meter true thermal expansion signals from the signals of the interference beam.
In the former case, disturbance oscillation, like atmospheric swing, results in fluctuation of the phase term .phi. in the expression (1). This will cause noise in measuring the phase term p(t), thereby causing the accuracy of measurement to be reduced.
As the latter method (i.e., the reflectivity measuring method) aims at measurement of temperature changes and plasma density changes of a sample, it is impossible to obtain thermo-elastic characteristics such as thermal expansion ratio, etc. of a sample. And as only the information in thermal diffusion length can be obtained thereby, the method has such a defect that the depths of a sample can not be evaluated. Furthermore, the method is basically applicable only to samples of which reflectivity can change due to the temperature change.